1. Field of the Invention
This invention relates to electrophotography. More particularly, it relates to chemical sensitization of photoconductive compositions and electrophotographic elements with chemical sensitizing polymers comprising repeating units containing highly chlorinated monovalent pendant radicals.
2. Discussion of Related Art
The process of xerography, as disclosed by Carlson in U.S. Pat. No. 2,297,691 (issued Oct. 6, 1942), employs an electrophotographic element comprising a support material bearing a coating of a normally insulating material whose electrical resistance varies with the amount of incident electromagnetic radiation it receives during an imagewise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge, generally in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of this surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or discharge pattern as desired. Deposited marking material can then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor, or the like, or transferred to a second element to which it can similarly be fixed. Likewise, the electrostatic latent image can be transferred to a second element and developed there.
Various photoconductive insulating materials have been employed in the manufacture of electrophotographic elements. For example, vapors of selenium and vapors of selenium alloys deposited on a suitable support, and particles of photoconductive zinc oxide dispersed in resinous, film-forming binder have found wide application in the present-day document copying applications. For example, in the photoconductive compositions of U.S. Pat. No. 3,008,825 (issued Nov. 14, 1961 to W. G. Van Dorn et al), inorganic photoconductors are dispersed in such binders as polymerized butyl methacrylates, or vinyl polymers such as polymers of styrene, vinyl chloride, vinyl acetate, and the like. In dispersing inorganic photoconductors in acrylic polymer binders, the technical literature indicates that it is preferred to employ acrylic terpolymers and acrylic polymers free of acid groups, owing to the lowering of light sensitivity when either acrylic homopolymers, acrylic copolymers, or acrylic polymers containing acid groups are employed. (See Photographic Science and Engineering, Volume 16, No. 5, September-October 1972, pp. 354-358, and in particular, pp. 355-357).
Since the introduction of electrophotography, a great many organic compounds have also been screened for their photoconductive properties. As a result, a very large number of organic compounds have been shown to possess some degree of photoconductivity. Many organic compounds have revealed a useful level of photoconductivity and have been incorporated into photoconductive compositions.
In photoconductive insulating compositions using organic photoconductors, the photoconductor, if not polymeric, is usually carried in a film-forming binder. Typical binders are polymeric materials having fairly high dielectric strength such as phenolic resins, ketone resins, acrylic ester resins, polystyrenes and the like. A more comprehensive listing of binders appears in U.S. Pat. No. 3,755,310 (issued Aug. 28, 1973 to L. J. Rossi). The photoconductor can be dissolved with the binder to prepare a homogeneous photoconductive composition in a common solvent. In another aspect, it can be provided as a dispersion of small particles in the binder to prepare a heterogeneous phtoocnductive composition. A general discussion of such dispersions and their preparation appears in U.S. Pat. No. 3,253,914 (issued May 31, 1966 to G. Schaum et al).
Organic photoconductors demonstrate widely varying degrees of solubility in the organic solvents used to dissolve many of the common binders. In the preparation of homogeneous photoconductive insulating compositions, organic photoconductors such as p-terphenyl and others of low solubility in popular solvents cannot usually be included in sufficient concentration to provide compositions of desirable light-sensitivity. By use of dispersion techniques such as those referred to in the case of zinc oxide photoconductors, heterogeneous photoconductive insulating compositions having higher concentrations of low solubility photoconductors can be obtained, the objective being to improve light-sensitivity in the composition.
Heterogeneous organic photoconductive compositions as discussed herein can be advantageous, especially in the preparation of electrophotographic elements on which visible images will be provided. For example, such elements are both lighter in weight than elements having inorganic photoconductors like zinc oxide, and can be prepared to resemble bond paper. However, they have not enjoyed in such applications the popularity of photoconductive insulating compositions comprising inorganic photoconductors. This is largely due to the unacceptable photoconductivity of heterogeneous compositions of organic photoconductors, despite high concentrations of photoconductor.
To improve the photoconductivity of photoconductive compositions having organic photoconductors, a variety of compounds and polymers have been studied for use as so-called chemical sensitizers or activators. When added to photoconductive compositions it is intended that such materials enhance the photoconductivity of the composition at least within the electromagnetic wavelength region in which the composition is intrinsically sensitive. If successful, the composition is said to be chemically sensitized or activated. It should be pointed out, however, that chemical sensitizers are oftentimes specific in their utility. That is, they may have utility in homogeneous systems or heterogeneous systems but not generally in both. Materials which do serve as sensitizers for both systems, accordingly, are rare and highly desirable.
In copending application U.S. Ser. No. 800,483, now U.S. Pat. No. 4,082,550 entitled HEXACHLOROCYCLOPENTENE CHEMICAL SENSITIZERS FOR HETEROGENEOUS ORGANIC PHOTOCONDUCTIVE COMPOSITIONS in the name of William E. Yoerger filed concurrently herewith, an invention is described wherein certain highly chlorinated compounds, in particular monomeric hexachlorocyclopentenes, are highly useful as chemical sensitizers for photoconductive compositions having cellulose nitrate as binder for dispersed organic photoconductors. The success of such monomeric sensitizers in photoconductive compositions is in part attributable to the presence of cellulose nitrate binder. As further pointed out in the aforementioned copending application, choice of binder in a heterogeneous photoconductive composition having dispersed organic photoconductor can affect the ability of the composition to be sensitized. In the case of acrylic polymer binder, this is especially true. Despite references in the prior art to compositions comprising polymers including acrylics as binders for dispersed organic photoconductors (see for example, British Pat. No. 1,431,943 published Apr. 14, 1976 and issued to the Dow Chemical Company) attempts to improve the photoconductivity of such compositions with known chemical sensitizers have been largely unsuccessful. This is unfortunate as acrylic polymers offer especially good hardness, a desirable characteristic in electrophotographic elements.
Selection of a proper chemical sensitizer is further complicated by other requirements of an electrophotographic process. An element employing a chemically sensitized photoconductive composition as defined herein must, for example, readily accept and hold electrostatic charge before imagewise illumination. Many compositions employing materials screened for use as sensitizers, although acceptably photoconductive, undesirably fail to accept a high enough charge to merit further study. Compositions so failing are said to be "charge saturated. Further, though able to accept charge, compositions may be unable to retain that applied charge for reasonable periods of time in the dark, hence the term "dark decay."